In color TV broadcasting standards, such as NTSC, PAL and SECAM, the transmitted signals include chrominance signals and luminance signals. In comparison to the luminance signal bandwidth, the chrominance signal bandwidth is rather narrow. The limited bandwidth of the chrominance signal produces relatively slow chrominance transitions, causing smeared color edges in the received/displayed images.
Different techniques have been used in attempts to enhance the sharpness of color transitions, such as those described in U.S. Pat. No. 4,935,806 and U.S. Pat. No. 5,920,357, known as “Color Transient Improvement(CTI)” techniques. The basic steps in the Color Transient Improvement techniques are to add a high-pass filtered signal, such as the second derivative of the original signal, to the original chrominance signals to restore the frequency components lost due to the limited bandwidth. This is followed by a post-processing to remove any undershoot and overshoot.
However, as shown in the example color transition plot in FIG. 1, the center of the color edge has a second derivative that equals zero, wherein the color transition near the center of the color edge is almost “flat”. Therefore, the high-pass filter response near the center of the color edge is very weak. As a result, the region near the center of the color transition is left barely touched by conventional Color Transient Improvement methods.
There is, therefore, a need for a video enhancement method that sufficiently sharpens the slope of the color transient, and for an apparatus that properly processes the “flat” region near the center of the color transition to restore the frequency components lost due to the limited bandwidth.